The measurement of the temperature of a semiconducting wafer belongs to the most important tools during quality control in semiconductor production. Usually the temperature is measured contactlessly by use of a pyrometer which is adapted to intercept and measure the thermal radiation which is emitted by the semiconducting wafer. Said thermal radiation is then used to calculate the temperature of the semiconducting wafer.
Conventional pyrometers are adapted to detect thermal radiation in the range of 700 nm to 12 μm, i.e. radiation in the infrared spectrum. That is, the conventional pyrometers are sensitive to infrared radiation. According to Planck's law, the intensity of thermal infrared visible radiation is significantly higher than the intensity of e.g. thermal ultraviolet radiation (also referred to as UV-radiation). In the sense of the present invention, UV-radiation is radiation in the range of 250-450 nm, more preferably 300-420 nm and still more preferably 400-410 nm.
Some semiconducting wafers, such as silicon-carbide, sapphire or gallium-nitride wafers, are transparent in the infrared spectrum and therefore do not emit infrared radiation. According to the present invention, the expression “transparent” is understood in that an optical transmittance is higher than 50%, preferably higher than 70% and more preferably higher than 90%. Such infrared-transparent wafers are very important for industrial production of optoelectronic devices. For example such wafers are needed for the production of blue and white light-emitting diodes (LEDs) as well as for the production of III-nitride power electronics based on silicon-carbide substrates. During manufacturing of such wafers the required accuracy of temperature control has to be better than ±1K. Due to the absence of significant infrared radiation from the infrared-transparent wafers, conventional pyrometers cannot be used for wafer temperature sensing. In some cases (SiC, GaN) the temperature is conventionally measured by detecting the band edge shift. However, said method of temperature measurement does not meet the above discussed accuracy requirements.